Published patent application US2009/0322208A1 discloses a light emitting device. A Light Emitting Diode (LED) is provided within a conical cavity formed by a recessed housing. At the front side of the recessed housing the conical cavity is covered with a transparent thermal conductor layer on which a refractory luminescent layer is provided. At the backplane of the recessed housing a heat sink is provided and the side walls of the recessed housing are covered with a metal frame. The conical cavity may be filled with a material such as silicone.
The LED emits light of a first color towards the luminescent layer. A portion of the emitted light may be reflected or scattered back into the cavity by the luminescent layer. Another portion of the emitted light is converted by the luminescent layer into light of a second color. When the luminescent layer emits the light of the second color, this light is emitted in all directions, and thus a part of the light of the another color is emitted into the cavity. Light which is reflected back into the cavity or light of the second color which is emitted into the cavity partially impinges on a base of the cavity, partially impinges on a wall of the cavity, and partially impinges on the LED. At the surfaces of the LED and at the surfaces of the cavity the light is partially reflected and partially absorbed. Especially the absorption of light results in an inefficiency of the light emitting device.
Some light module manufacturers provide light emitting modules which comprise a cavity with a base. These modules often have a plurality of light emitters, such as for example LEDs, provided on the base. In certain embodiments of these light emitting modules the luminescent layer is provided directly on top of the light emitters, for example via a bond layer, and in other embodiments the luminescent layer is a so-called remote luminescent layer which means that there is a relatively large distance between the light emitter and the luminescent layer in the order of centimeters.
A problem of the light emitting modules with the light emitters which have the luminescent layer directly on top is that light directed back from the luminescent layer to the LED suffers from poor recycling efficiency due to the fact that back reflectors inside the LED have a limited reflectance (typically the back mirror is silver, with 90% reflectance levels). In reality the actual reflectance is even lower as the light emitter material, typically GaN/InGaN or AlInGaN, has a high refractive index, causing light to be trapped inside the light emitter and thus further limiting metal reflectance. Typical LED reflection coefficients are close to 70% (averaged over the visible spectral range and measured at normal incidence). Another problem of these light emitting modules is the formation of hot spots in which most of the light is concentrated in the area on top of the LED and the light output of the module is therefore highly non-uniform resulting in hot spots both in light output and thermal distribution. Furthermore, a phosphor layer on top of the LED die may get relatively hot and is excited with a high flux density, leading to a non-optimal phosphor conversion efficiency thereby limiting the luminescent performance.
The light emitting modules with the remote luminescent layer are generally more efficient than the light emitting modules with the light emitters which have the luminescent layer directly on top, because of a more efficient recycling of light inside the cavity. Also the light output of these modules is typically more homogeneous, reducing the hot spots. However, the light emitting modules with the remote luminescent layer have a relatively large size compared to the light emitting modules with the light emitters which have the luminescent layer directly on top. The relatively bulky remote luminescent layer solutions cannot be used in size constrained applications, such as spot lamp applications, for example halogen replacement lamps and parabolic reflector lamps.
Another disadvantage of light emitting modules with a remote luminescent layers is that the relatively large area of the luminescent layer results in relatively high material cost levels. In addition, the heat conductance within the phosphor layer is only directed laterally towards the side walls of the light emitter and due to their bulky construction, the ability to direct the heat away from the remote phosphor plate is limited.